1. Field of the Invention
The present invention relates to a power amplifier. More particularly, the present invention relates to an apparatus and a method for improving the efficiency of a power amplifier which operates with a signal having a high peak-to-average power ratio.
2. Description of the Related Art
In a conventional radio communication system such as a cellular system, a base station uses a power amplifier to communicate with user terminals which are located within a predetermined service area. The power amplifier amplifies and transmits radio frequency (RF) signals used in a communication system for transmitting multiple signals.
Specifically, in a cellular system using a code division multiple access (CDMA) technique, an orthogonal frequency division multiplexing (OFDM) technique or the like, it is necessary owing to its particular characteristics to transmit modulated multiple signals having a high peak-to-average power ratio (‘PAR’) to multiple users or user terminals sharing the same frequency band. Conventional power amplifiers used to amplify a radio frequency signal in such a communication system results in the consumption of a lot of power because it is necessary to amplify and transmit a signal having a high PAR, which is inefficient and increases the manufacturing cost.
A peak reduction scheme for reducing a peak value of a signal is typically used to decrease the average power loss in a power amplifier which operates with a signal having a high PAR. Such schemes include “Clipping Noise Mitigation for OFDM by Decision-aided Reconstruction”, to Kim et al, IEEE Communications Letters, Vol. 3, No. 1 January 1999 and “Design Considerations for Multicarrier CDMA Base Station Power Amplifiers”, to J. S. Kenney et al, Microwave Journal, February 1999 which are incorporated herein by reference.
FIG. 1 is a block diagram illustrating an apparatus for improving the efficiency of a power amplifier using a conventional peak reduction scheme.
According to a conventional peak reduction scheme, when an input signal having a high PAR is input from a base band source signal generator (‘BSG’) 110 to a peak-reduced signal generator (‘PSG’) 120, the PSG 120 detects a peak signal of the input signal and reduces the level of the detected peak signal, thereby outputting a signal having a reduced PAR. A signal output from the PSG 120 is coupled to a local oscillation signal, which is output from an RF local oscillator (‘RFLO’) 135, by a quadrature modulator (‘QM’) 130, thereby being converted into a quadrature-modulated RF signal. An RF power amplifier (‘RFPA’) 140 amplifies the quadrature-modulated RF signal output from the QM 130 and transmits the amplified signal to an antenna.
According to the conventional peak reduction scheme, as the PAR of a signal becomes lower, the efficiency of the power amplifier improves, but an error rate and spectral re-growth of the signal increases, so that a signal characteristic is significantly deteriorated. That is, when the peak reduction scheme is used, there is a limitation in improving the efficiency of a power amplifier. Therefore, it is necessary to develop a technique capable of effectively improving the efficiency of a power amplifier without increasing the error rate of a signal.